One of the primary disadvantages of replacing coal with charcoal in metallurgical or chemical processes is that charcoal has a substantially lower density than comparable coal products. In order to make charcoal more attractive for industrial uses, a method of increasing the charcoal density, while maintaining an economically attractive price, needs to be developed. Ideally, charcoal density needs to be increased by a factor of 2-3 times from a range of 0.2-0.5 g/cm3 (this density range is common to charcoal produced using conventional methods from typical wood sources) to a density of 0.6-1.0 g/cm3.
Producing charcoal with the density greater than charcoal made from conventional raw materials and by a conventional pyrolysis process has been proposed by:    (1) densification of raw materials heated up to 220° C.-250° C. with their subsequent conventional pyrolysis, by for example the use of high-density wood pellets (HDWP) or extruded bars or    (2) pyrolysis of conventional or densified raw materials under constant or pulsating pressure externally applied to thermally decomposing material throughout most of the duration of pyrolysis process.
Methods (1) above, usually employs compression of sawdust, preheated up to 220° C., or sometimes even to 250° C., by pressures in excess of 2,000 bars (e.g. for the pellet machine the pressure may vary between 2,000 bars and 4,500 bars). The process includes converting wood or other organic material into sawdust, conditioning it to the required moisture content and temperature, pelletising or extruding it at high pressure and then cooling it down to the ambient temperature. Subsequently, the densified material undergoes a conventional pyrolysis. Equipment used for such a densification of raw materials is relatively complex and in the case of extruded bars, has a limited productivity.
For this group of methods, no large-scale commercial operations, which could satisfy the needs of metallurgical, chemical or power industries, are known. The use of densified wood products to make high quality charcOal for domestic needs or as an absorbent has been occasionally reported.
Despite the high density of the initial material (e.g. up to 1.3 g/cm3 for HDWP) the density of charcoal obtained from it was modest, around 0.7 g/cm3, because gases and vapours released during the pyrolysis of such a densified material create pores when escaping from the core of the reacting particles.
Method 2 above, includes US patents by Hawley (Hawley, L. F. The Production of Artificially Dense Charcoal. The Journal of Industrial and Engineering Chemistry (1921), April, pp. 301-302, U.S. Pat. Nos. 1,369,428, 1,385,826) and Danilov (Russian patent 2217468). In the Hawley patents artificially dense charcoal is produced from sawdust or sawdust briquettes compressed at ambient temperatures using a pressure of at least 2000 bars. The sample was placed in a 2.5″ ID externally heated steel pipe and was subjected to pressure exerted by a plunger throughout the duration of the pyrolysis reaction. The applied pressure was either constant or oscillating in nature. In particular, Hawley determined that an oscillating pressure varying between 3.5 bars and 8.5 bars resulted in a final product with a density of at least 0.95 g/cm3.
Hawley also determined that to obtain dense charcoal the pressure applied to the material undergoing pyrolysis may be up to three orders of magnitude lower than that applied to the material at temperatures of up to 220° C. to achieve compression. This effect was associated with a softening of ligno-cellulosic material at the higher temperatures, and therefore a reduction in its mechanical strength, allowing greater compression. Hawley concluded that for common wood species substantial softening occurs within the temperature range of 280° C. to 300° C. (U.S. Pat. No. 1,369,428). Pressure applied to ligno-cellulosic material softened by heating to temperatures in this range, results in greater compression of the material. Hence densified charcoal obtained this way was found to be more dense (less porous) than charcoal obtained by the conventional pyrolysis of densified raw materials (e.g. HDWP).
The process described by Hawley is a batch process. No commercial densified charcoal production methods using this process are known.
Danilov (Russian Patent 2217468) proposed a continuous process for production of dense charcoal. In this process ligno-cellulosic material (preferably sawdust) was initially densified in a screw extruder at temperatures of up to 280° C. and pressures of up to 1200 bars. The material was then directed in to a tubular pyrolysis reactor where it was further compressed by a plunger pushing down on the material and pressurised by pulses of hot high-pressure gases entering the reactor through perforated side walls. The reaction vessel required external heating. No commercial densified charcoal production methods using this process are known.
The methods developed by both Hawley and Danilov had several limitations, in particular the gas permeability of the material became negligible as a result of material densification, therefore the reactor required external heating, as heating the material by hot gas flowing though it was impossible. Furthermore, since the thermal conductivity of the ligno-cellulosic material was very low, the size of the reaction vessel that could be used was limited.
In contrast to the processes described by Hawley and Danilov, where external heating was required, pyrolysis can proceed in a fully autogenous mode, i.e. no external heat is needed, as all the heat required by the process is generated by the reactions occurring with the material itself. In respect to the production of densified charcoal, the use of this autogenous process provides a major advantage in that it allows the temperature distribution across any transverse cross-section of the reaction vessel to be reasonably uniform.
Reference to any prior art in the specification is not, and should not be taken as, an acknowledgment or any form of suggestion that this prior art forms part of the common general knowledge in Australia or any other jurisdiction or that this prior art could reasonably be expected to be ascertained, understood and regarded as relevant by a person skilled in the art.